AQUEOUS ALTERATION IN 5 CHONDRITIC POROUS INTERPLANETARY DUST PARTICLES

Analytical electron microscope analysis of stratospheric micrometeorites show that layer silicates are common, albeit minor, constituents in five particles of the nominally anhydrous subclass of chondritic porous (CP) interplanetary dust particles (IDPs). These layer silicates are single-crystal Na,K-rich smectite minerals, illite, kaolinite and less-common pyrophyllite, mica and rare talc. Rare, non-stoichiometric crystalline feldspars of probable authigenic origin occur in CP IDP W7029E5. The feldspars are low-anorthite and undetermined (Na,K)-alkali feldspar polymorphs. Similar Al2O3/(Al2O3 + SiO2) ratios and alkali content of feldspars and layer silicates suggest a genetic relationship between these minerals whereby layer silicates form during aqueous alteration. In the case of CP IDP W7029E5 alteration occurred in a mildly acidic (pH = 5-7) fluid at 175-195-degrees-C. If CP IDPs are solid debris from short-period comets, the observations give evidence for mineralogical and chemical activity in nuclei of short-period comets. If an asteroidal origin for these CP IDPs can be established, these observations provide a window to the processes that may ultimately lead to fully hydrated carbonaceous chondrite matrices.